1. Field of the Invention
This invention relates to a method and apparatus for the growth of an oxide single crystal or a semiconductor single crystal by the Czochralski method (hereinafter referred to as the CZ method) or the liquid encapsulated Czochralski method (hereinafter referred to as the LEC method). In particular, it is concerned with a method and apparatus for the Czochralski growth of gallium arsenide (GaAs) bulk single crystals to be used for integrated circuits, electronic devices for communication, optical devices, etc.
2. Description of the Related Art
Generally, a single crystal bulk such as composed of GaAs is grown by the CZ method or LEC method, but there arises a problem that polycrystals partly occur during the growth.
FIG. 10 is a schematic view of an apparatus for carrying out the LEC method in the prior art. At the center of a high pressure chamber 13, a susceptor 14-fitted crucible 1 is supported by a lower shaft 6. The crucible 1 holds a raw material melt 3 and a liquid encapsulant 5. A seed crystal 9 held by an upper pulling rod 7 is immersed in the raw material melt 3, followed by pulling a single crystal 4. A heater 10 is arranged around the raw material melt 3. A heater 11 is arranged around the single crystal 4 and a heat insulator 12 is arranged inside the high pressure chamber 1.
The reason for the polycrystallization, as described above, is considered to consist in that the shape of the solid-liquid interface (S-L) of the growing crystal is concave to the melt near the periphery of the growing crystal. Dislocations (D) propagated along a direction perpendicular to the solid-liquid interface accumulate in the periphery, thus rendering polycrystalline (P), as shown in FIG. 11. In order to prevent formation of such polycrystals, therefore, it is necessary to prevent the solid-liquid interface near the periphery of the growing crystal from becoming concave. As a method of preventing the solid-liquid interface from becoming concave in the CZ method or LEC method, it has been proposed to increase the power of a heater arranged in the vicinity solid-liquid interface as much as possible to locally heat the side of the growing crystal and to suppress heat flow laterally from the crystal HITACHI CABLE REVIEW No. 9 (1990), 55!.
However, the above described method has the following problems:
1. In order to locally heat the side of the growing crystal, it is necessary to shorten the length of the heater to some extent and apply a larger power. Consequently, the current density flowing through the heater is very large and the service life of the heater is very short.
2. Since heating of the heater is carried out through a susceptor made of carbon, the crystal is uniformly heated in the vertical direction by heat conduction and substantial local heating is impossible. In the case of the LEC method, furthermore, B.sub.2 O.sub.3 used as an encapsulant absorbs the most part of heat radiation and accordingly, it is impossible to effectively heat the most important region, i.e. in the vicinity of the solid-liquid interface.
3. Since not only the periphery part of the crystal but also the whole of the crystal side surface are heated, heat flow toward the side from the crystal is wholly suppressed and the order of convex to the melt of the solid-liquid interface increases, so large residual strain is contained in the growing crystal and uniformity of the property of a wafer cut out of the crystal is poor.
4. In the CZ method or LEC method, controllability of the diameter of the growing crystal is strongly dependent on the temperature distribution on the surface of the raw material melt. In the prior art, however, it is difficult to stably maintain the above described temperature distribution with good reproducibility and when using a large amount of a raw material at once and using a crucible with a large diameter for high productivity of the single crystal, in particular, control of the diameter further becomes difficult with an increase of the gap between the growing crystal and crucible wall.
5. The LEC method is ordinarily used for the growth of a single crystal containing a volatile element, but in this method, the surface temperature cannot be so raised because a growing crystal pulled out of the liquid encapsulant is damaged through evaporation of the volatile element from the surface. Consequently, the temperature gradient in the axial growth direction is restricted to a higher region (temperature gradient in growth of GaAs single crystal: ordinarily about 100.degree. C./cm) and a crystal with many crystal defects (dislocations) and much residual strain can only be grown because of the occurrence of large thermal stress in the growing crystal.
6. Thus, the growth is carried out using a large amount of a liquid encapsulant so that most of the growing crystal is covered with the liquid encapsulant, in order to practice the LEC method under a small temperature gradient (hereinafter referred to as Fully Encapsulated Cz method, i.e. FEC method). But because of the very small heat conductivity of the liquid encapsulant B.sub.2 O.sub.3, response of the temperature on the surface of the raw material melt is deteriorated and consequently, control of the diameter of the crystal is extremely difficult. This cannot therefore be said to be a useful growth method.